The concept of butterfly valves with a central pivotable disk is known in the art. One such butterfly valve is shown in Steele et al. U.S. Pat. No. 4,836,499 and another is shown in Steele U.S. Pat. No. 5,295,659. In both valves a central disk pivots from a transverse or closed position, where the disk extends across a flow passage to block materials from passing through the valve, to an axial or open position, where materials enter one side of the valve and flow laterally alongside the disk faces before being discharged from the opposite side of the valve. To seal the valve the disk is positioned in the transverse position and an annular elastomer seal or sealing ring is inflated to engage an edge of the disk to form an annular seal around the periphery of the disk. To pivot the disk to an open condition or flow through condition the elastomer seal is deflated and the disk is pivoted from the closed position to an open position.
One of the difficulties with butterfly valves is obtaining a high-pressure seal between the annular edge of the pivotal disk and an inflatable seal which will not leak as the pressure differential across the disk increases.